Capacitor foil making



United States Patent Office 3,346,424 Patented Oct. 10, 1967 3,346,424 CAPACITOR FOIL MAKING Donald W. Waiters, Ravenswood, W. Va., assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Caliii, a corporation of Delaware No Drawing. Filed Dec. 16, 1964, Ser. No. 418,898 3 Claims. (Cl. 148-2) This invention relates to aluminum material for use in the manufacture of capacitors. More particularly, it relates to an improved method of processing molten aluminum metal for subsequent use in making rolled stock, usually foil, for use as electrodes for capacitors, particularly electrolytic capacitors.

In the manufacture of electrolytic capacitors or condensers, at least one of the electrodes is of a film-forming metal, that is, a metal which has the capability of being electrolytically oxidized with a film having high dielectric properties. Aluminum is the most commonly used material for these electrodes, and as impurities in the metal are detrimental to the dielectric properties of the aluminum oxide layer, aluminum of high purity, e.g., aluminum containing less than a few hundreds of a percent of impurities, is required for high capacitance characteristics and satisfactory long life.

Many complex factors in the processing of the aluminum metal, some of which are interrelated to the purity of the aluminum, are important in the making of a satisfactory material for electrolytic capacitors. An important requirement is that the aluminum material have a large reactive surface area and to obtain this the aluminum surfaces must be uniformly etched in a suitable etchant prior to the film formation. Successful etching of the aluminum depends upon the grain structure of the metal which in turn depends upon the purity and the character and amount of the impurities in the metal. The processing of the metal, i.e., melting, fluxing the molten metal, and casting into ingot form and the subsequent steps of hot rolling, cold rolling (including rolling to foil gauge) and annealing are all interrelated and must be carefully and precisely controlled to produce a foil product which, when etched and electrolytically oxidized, exhibits high dielectric properties.

The melting, fluxing and casting of the high purity aluminum are of prime importance in the obtaining of a satisfactory capacitor foil. Without laying the proper foundation in processing the molten aluminum into ingot, the subsequent steps, such as rolling, annealing, etc., even if properly and adequately performed, will not produce a premium foil product which will exhibit high capacitance after etching and film forming.

By the instant invention there is presented an improved method of melting, fluxing and casting which will yield a foil material, which after etching and film forming has a higher capacitance and a larger shelf life than material heretofore manufactured.

The instant invention comprises generally the steps of melting high purity aluminum metal charged into a suitable furnace, fiuxing the metal with chlorine (C1 gas for a period of time to substantially degas the metal and to remove deleterious impurities, said fluxing commencing immediately after the metal charge has become molten, and thereafter, while still continuing chlorine fiuxing of the metal, immediately casting the metal into ingot form by continuous casting. The fiuxing and casting operations should be completed within 120 minutes, preferably within 60 minutes. The time for the total cycle of melting, fluxing, and casting should not exceed 200 minutes. The metal temperature during processing should be maintained in the range of about 1290l330 F., preferably in the range of 1300-1315 F.

It is important that the metal throughout the operation have a minimum amount of superheat, that is, heat above that required for melting. As the absorption of gas, particularly hydrogen, by the molten aluminum is directly proportional to the temperature of the metal, the amount of superheat in the molten metal should not substantially exceed that amount of heat which is lost to transfer mechanisms, such as troughs, etc. when the molten metal is transported from the furnace to the mold.

The invention has particular importance to high purity aluminum in which the purity is 99.99% or higher, for example, 99.992% and 99.996%. In regard to electrolytic capacitors, the aluminum foil used for the anode or the positive plate must be of the highest purity obtainable. Aluminum is advantageous for electrolytic capacitor manufacture not only for its good electrical conductivity, but also for its capability of having formed on its surface a thin coating of aluminum oxide, which is a superior dielectric layer. The dielectric constant of aluminum oxide is higher than that of wax paper and many other dielectric materials which are used in fixed type capacitors, therefore, the oxide dielectric plate, which determines the size of the given capacitor, does not have to be as large, for example, as a paper plate for equal capacity. Further, the aluminum oxide film, even a relatively thick one, is thinner than the thinnest tissue paper. This thin film of aluminum oxide reduces the bulk of the capacitor, but more important, its thinness also increases the capacitance per unit of area. The reason for employing the highest purity of aluminum available is not to gain greater conductivity, but rather to obtain the purest possible aluminum oxide film. Any impurities in the film will permit electrons to leak at those spots which would distort the current flow and heat up the capacitor thereby reducing its life.

The purity of aluminum produced in electrolytic reduction cells of the Hall-Heroult type is limited by the impurities in the alumina, the electrolyte materials and electrode carbon; consequently, the purity of the aluminum produced in the cell is normally only in the range of about 99.5-99.8% aluminum. To obtain metal of a higher purity, that is, 99.99% and higher, a refining process must be employed. Refining of impure aluminum is generally carried out electrolytically by the three-layer refining process wherein the electrolysis medium includes an anode layer containing impure aluminum alloyed with a heavier metal, such as copper, a cathode layer which is pure aluminum, and a molten salt electrolyte layer which has a density less than that of the anode layer but greater than that of the purified aluminum layer. During electrolysis, the aluminum is transferred from the lower anodic molten layer of impure aluminum and heavier metal to the top cathodic layer of pure aluminum via the intermediate layer of molten electrolyte. The Chemical Background of the Aluminum Industry, T. G. Pearson, The Royal Institute of Chemistry, Lectures, Monographs and Reports (1955), No. 3, pp. 64-72, gives an informative review of the three-layer refining process.

The fiuxing with chlorine gas is accomplished by means of any suitable fiuxing apparatus, such as the wand type apparatus comprised of an L-shaped steel tube. One end of the wand apparatus is connected to a suitable source of chlorine gas While the other end during fluxing is submerged beneath the molten aluminum for emission of the chlorine purge gas into the body of molten metal. During the fluxing operation, it is desirable that the metal be disturbed as little as possible by mechanical stirring. In order to accomplish this, a plurality of wands may be placed in the furnace so that all or substantially all of the molten aluminum is intimately exposed to the action of the chlorine gas. As noted previously above, this fluxing operation should be for a period of 10 to 15 minutes. During the continuous casting of the metal into an open end mold while simultaneously cooling the emerging ingot by direct water chilling, the metal which is withdrawn from the furnace or receptacle should be continuously fluxed with chlorine gas during the casting operation. This can be accomplished by disposing a flux wand or wands in close proximity to the taphole or outlet of the furnace or preferably the furnace can be equipped with a flux bay arrangement which is provided with a plurality of chlorine flux tubes, such as is disclosed in U.S. 2,821,472, W. S. Peterson et al., issued I an. 28, 1958. The purpose of fluxing during the casting operation is to remove any reabsorbed gas, e.g., hydrogen, that may occur subsequent to the previous fluxing in the furnace and prior to release from the furnace.

For the purpose of illustrating the invention and its advantages, the following preferred procedure for carrying out the instant invention is set forth:

The charge to the furnace or receptacle should be high quality aluminum, e.g. aluminum pig of 99.99% purity. Aluminum metal of this quality would have a composition by weight as follows:

Percent Fe 0.001-0.002 Si 0002-0003 Ca. 0.00l0.002 Mg 0002 0.003 Al Remainder In order to make a high quality capacitor foil stock the minor impurities in the charged metal should be low as evidenced by the composition shown above. Further, for best results, the entire charge should be of the high quality aluminum. Charging of scrap material, particularly oily scrap, is not recommended because control over minor impurities is lost resulting in a lack of grain size control. Oily scrap would result in an increased gas content in the molten metal.

In the melting, fluxing, and casting of the metal, the following conditions should be adhered to:

(1) Maximum molten metal temperature 1330 F.

(2) Maximum total time for fluxing and casting-120 minutes, preferably 60 minutes.

(3) The molten metal should not be disturbed by mechanical stirring or by use of a salt fluxing procedure.

(4) Only chlorine gas should be used for fiuxing and the chlornie should be introduced into the molten metal with a minimum amount of stirring.

As soon as the metal becomes entirely molten, the chlorine gas is bubbled into the molten metal for a period of to minutes, after which the metal is immediately cast into ingot form by the continuous casting process. The chlorine gas fluxing is continued throughout the casting of the ingot. Maximum casting speed should be used consistent with obtaining good-as-cast ingot surfaces.

After casting, the ingot should be subjected to a homogenizing heat treatment, for example, a 24 hour soak at 1130 F. After the homogenizing treatment, the ingot should be air cooled after which the surfaces of the ingot which will be in contact with the work rolls during the rolling operation are scalped or machined to remove surface defects.

For best results, the scalping or machining of the surfaces should be done not more than 24 hours before the hot rolling. For hot rolling, the ingot should be heated to about 932 F. and then hot rolled to a final gauge of approximately 0.235 inch. Following hot working, the metal is then subjected to cold rolling with suitable intermediate annealing between various stages of the rolling, for example, rolling to a thickness of about 0.05 inch after which the metal is annealed at 780 F. for 6 hours followed by further cold rolling steps to the final condenser foil gauge. The coils of rolled foil metal are slit to the finished size and then subjected to a recrystallization annealing procedure; for example, soaking at 932 F. for 12 to 18 hours.

High purity aluminum foil prepared according to the above procedure exhibited high capacitance measurements. These measurements which are set forth below in Table I were compared to the values obtained on high purity foil which had not received the molten metal processing, that is, the melting, fiuxing and casting, of the invention. In the making of this latter foil, the high purity aluminum charge was melted in a melting hearth and then was transferred while molten to a second hearth, a holding hearth, for chlorine gas fluxing with two wands disposed on the hearth for a period of about five minutes. During casting, the metal was passed through a flux bay wherein chlorine flux tubes emitted chlorine gas into the molten metal during the entire casting operation. In this melting, flux and casting operation, no particular effort was made to flux immediately as soon as the entire melt became molten or to cast the molten metal as soon as the metal had been fluxed. Samples of this foil, as well as samples of foil made according to the invention, were subjected to a standard 500 volt capacitance test used for testing electrolytic capacitor foil wherein the foil is subjected, in successive steps, to an electrolytic etching in an NaCl solution (812% NaCl), to a film-forming procedure to form an anodized coating on the aluminum sample in a boric acid bath, e.g., at a voltage of 500 volts and then to a capacitance measuring procedure using a suitable capacitance test bridge and a solution of boric acid and sodium tetraborate. With an electrolytic etching procedure, employing a 12% NaCl solution, and various precleaning methods prior to electrolytic etching, the capacitance results of the standard 500 volt test are as follows:

TABLE I High Purity Foil High Purity Foil Not Prepared Prepared According According to the To the Invention Invention Precleaning Treatment Capacitance, Capacitance, Microfarads/5 sq. in. mid/5 sq. in.

(mid/5 sq. in.)

1 No precleaning Material failed due 6. 35

treatment. to perforation. 2 NaOH pre- 6.2

cleaning. 3 Borax predo 6.4

cleaning.

Using a lower concentration of NaCl in the electrolytic etching procedure; i.e., 0.8% NaCl, the capacitance measurement of the foil prepared according to the invention exhibits values ranging from 6.5 to 7.0 mfd./5 sq. in. compared to values for the foil made not according to the invention of 5.5 to 5.6 mfd./ 5 sq. in.

The above figures, particularly those in Table I wherein the high concentration of NaCl is employed, show the efiicacy of the instant invention in improving the capacitance of high purity foil. Although the invention has particular application to high purity aluminum foils (1199) which are required for electrolytic capacitors, it can also be used to improve the properties and characteristics of lower purity aluminum foils, such as those containing 99.88% A1 (1188) and 99.80% A1 (1180).

It is to be understood that various changes and modifications may be made in the foregoing without departing from the spirit of the invention and the scope of the appended claims, wherein:

What is claimed is:

1. In a process for preparation of cast aluminum ingot stock for capacitor foil, the steps comprising charging a furnace with a quantity of high purity aluminum metal, melting said quantity of metal, fluxing the metal in the melting furnace with chlorine gas as soon as the metal has melted, said fluxing being performed so as to intimately expose substantially all the molten aluminum to the action of the chlorine gas and being for a period of time ranging from about 10 to 15 minutes and the chlorine gas being in an amount sufficient to supply a chlorine gas cover above the surface of the metal that is slightly above atmospheric pressure, and thereafter, while maintaining said fluxing with chlorine gas, immediately casting the metal into ingot form by continuously casting the metal into an open-ended mold while simultaneously cooling the emerging ingot by direct Water chilling, the temperature of the metal during melting, fluxing and casting being maintained at a temperature in the range of about 12901330 F. and the total time for the melting, fluxing and casting operations being not greater than about 200 minutes.

2. A process according to claim 1 wherein the time for the fluxing and casting operations is not greater than 60 minutes.

3. In a process of making aluminum capacitor foil wherein high purity metal is melted and cast into ingot form by continuous casting, and the ingot is then subjected to a homogenizing heat treatment, hot and cold rolled to foil gauge thickness and then subjected to a recrystallization anneal, the improvement comprising, in the processing of molten aluminum into ingot form, the steps of charging a furnace with a quantity of high purity aluminum metal, melting said quantity of metal, fluxing the metal in the melting furnace with chlorine gas as soon as the metal has melted, said fluxing being performed so as to intimately expose substantially all the molten aluminum to the action of the chlorine gas and being for a period of time ranging from about 10 to 15 minutes and the chlorine gas being in an amount sufiicient to supply a chlorine gas cover above the surface of the metal that is slightly above atmospheric pressure, and thereafter, while maintaining said filming with chlorine gas, immediately casting the metal into ingot form by continuously casting the metal into an open-ended mold while simultaneously cooling the emerging ingot by direct Water chilling, the temperature of the molten metal during processing of the molten metal being maintained at a temperature in the range of about 1290-1330 F. and the time for the melting, fluxing and casting operations being not greater than about 200 minutes.

References Cited UNITED STATES PATENTS 905,758 12/1908 Strange et a1. 22-2001 2,206,430 7/1940 Robinson 1753 15 2,511,775 6/1950 Kelly et a1. -68 2,616,799 11/1952 Bugel 75-148 2,821,472 1/1958 Peterson et a1. 7593 3,149,960 9/1964 Robinson 7568 I. SPENCER OVERHOLSER, Primary Examiner.

V. K. RISING, Assistant Examiner. 

1. IN A PROCESS FOR PREPARATION OF CAST ALUMINUM INGOT STOCK FOR CAPACTOR FOIL, THE STEPS COMPRISING CHARGING A FURNACE WITH A QUANTITY OF HIGH PURITY ALUMINUM METAL, MELTING SAID QUANTITY OF METAL, FLUXING THE METAL IN THE MELTING FURNACE WITH CHLORINE GAS AS SOON AS THE METAL HAS MELTED, SAID FLUXING BEING PERFORMED SO AS TO INTIMATELY EXPOSE SUBSTANTIALLY ALL THE MOLTEN ALUMINUM TO THE ACTION OF THE CHLORINE GAS AND BEING FOR A PERIOD OF TIME RANGING FROM ABOUT 10 TO 15 MINUTES AND THE CHLORINE GAS BEING IN AMOUNT SUFFICIENT TO SUPPLY A CHLORINE GAS COVER ABOVE THE SURFACE OF THE METAL THAT IS SLIGHTLY ABOVE ATMOSPHERIC PRESSURE, AND THEREAFTER, WHILE MAINTAINING SAID FLUXING WITH CHLORINE GAS, IMMEDIATELY CASTING THE METAL INTO INGOT FROM BY CONTINUOUSLY CASTING THE METAL INTO AN OPEN-ENDED MOLD WHILE SIMULTANEOUSLY COOLING THE EMERGING INGOT BY DIRECT WATER CHILLING, THE TEMPERATURE OF THE METAL DURING MELTING, FLUXING AND CASTING BEING MAINTAINED AT A TEMPERATURE IN THE RANGE OF ABOUT 1290-1330*F. AND THE TOTAL TIME FOR THE MELTING, FLUXING AND CASTING OPERATIONS BEING NOT GREATER THAN ABOUT 200 MINUTES. 